1. Field of the Invention
The present invention relates to a scanning optical system, for example, to a scanning optical system for use in apparatuses such as film scanners capable of high-speed image capture.
2. Description of the Prior Art
Various types of film scanners have been proposed. Of them, a film scanner of mirror-scan type is well known. The mirror-scan type film scanner is constituted of a line sensor (e.g. line charge coupled device (CCD)) having its light receiving devices arranged in a sub scanning direction, a scanning optical system for imaging film images on the line sensor, and a mirror being swingingly rotated for main scanning.
The above-described type of film scanner faces a problem that since the film image plane to be scanned is flat, when it is scanned, the optical path length between the mirror and the scanned image plane changes as the mirror rotates. To solve this problem, Japanese Published Patent Application No. S62-20526 discloses a scanning apparatus which achieves high-speed scanning of flat image planes without causing any curvature by disposing a rotationally asymmetrical imaging optical system having a desirable Petzval sum between the mirror and the scanned image plane to correct the optical path length.
However, the imaging optical system used in the scanning apparatus of Japanese Published Patent Application No. S62-20526 is an expensive optical system having a surface configuration which is difficult to manufacture, so that the cost of the scanning apparatus rises. In addition, since it is inevitable to use a large-size mirror, it is difficult to rotate the mirror at high speed, so that it takes ten seconds to several minutes to capture the image of one frame of the film.
In the scanning optical system of mirror-scan type, the mirror is swingingly rotated for scanning, so that a biased load is imposed on the bearing of the mirror every time scanning is performed. As a result, the bearing of the mirror is biasedly worn or partially out of oil. In addition, the driving apparatus (e.g. galvanic apparatus) for swingingly rotating the mirror is expensive and is a cause of the complication of the scanning apparatus.